In seawater desalination plants, which are plants which typically perform seawater intake, reverse osmosis methods which employ reverse osmosis membranes have become commonly used in recent years, instead of evaporation-type water treatment methods. In such reverse osmosis methods, clean seawater with fewer contaminants is sought through pre-treatment to produce fresh water, in order to reduce poor performance resulting from fouling (clogging) of reverse osmosis membranes by contaminants.
As shown in FIG. 7, in an example of a present seawater intake method, a direct water intake method is used in which seawater is taken in from a water intake orifice 1 via a water conduit 2 provided on the ocean floor. In FIG. 7, Reference Numeral 3 is a pump for taking in the seawater, and Reference Numeral 4 is a reverse osmosis membrane system.
However, when employing the direct water intake method, debris, sediments, and living organisms are all taken in at the same time as the water, and thus there are cases in which water intake has to be stopped, for example, when there is abnormal adhesion of jellyfish or algal blooms, oil spill accidents, and increased turbidity due to high waves. Moreover, when employing the direct water intake method, it is necessary to perform periodic cleaning, to add chemicals such as chlorine to prevent adhesion, or to increase the diameter of pipes when living organisms becoming attached to the entire length of the pipes are taken into consideration, because the adhesion of sea life such as barnacles and mussels can be significant. Furthermore, when chlorine is introduced into seawater to prevent the adhesion of marine life when employing the direct water intake method, there arise problems of environmental pollution and biofouling of reverse osmosis membranes, which are remotely caused by chlorine. In addition, when intake seawater is treated by reverse osmosis, a sand filtration system must be installed for filtering seawater to which a coagulant has been added, and thus there is a need to install a system for treating sludge which accumulates in the sand filtration system.
Accordingly, in recent years, attention has been focused on indirect water intake methods which take in seawater from a sand layer 5 on the ocean floor, as shown in FIG. 8, without using chemicals such as coagulants to pre-treat the intake seawater.
As illustrated in FIG. 9, an indirect water intake method is a method which involves excavation of an ocean floor at an offshore site several hundred meters from a shoreline and at a depth of several tens of meters, forming a sand filtration layer 5 from supporting gravel layers 5a and 5b, and a filtration sand 5c, and implementing backfilling up to the same ocean floor surface to install an intake pipe 6 in the supporting gravel layer 5a, from which seawater which is purified by infiltration is taken in. Although none of the problems of the direct water intake method arise when this indirect water intake method is employed, there are problems such as initial high cost and reduced water intake volume due to clogging when clogging substances (e.g., silt) become trapped in the surface layer of the sand filtration layer, and consequently, this method has been slow in achieving widespread use.
In a specific example of an indirect water intake method, the seawater infiltration rate achieved in the sand filtration layer on the ocean floor is set at 1-8 m/day, and it is also characterized in that the water depth of the sand filtration layer is greater than the critical water depth for total sediment movement at which sand in the surface layer portion of the sand filtration layer travels at least 50 cm, and less than the critical water depth for surface layer movement at which the sand travels at least 1 cm (Patent Reference 1). However, in the water infiltration intake method disclosed in Patent Reference 1, a large surface area is needed for the intake of a large volume of seawater in a short period of time, because the seawater infiltration intake rate of 1-8 m/day is a very slow filtration rate, and therefore requires a large-scale construction and increased cost.
In addition, since the water infiltration intake method disclosed in Patent Reference 1 utilizes natural waves or currents to remove clogging substances (e.g., silt) which accumulate in the surface layer of the sand filtration layer, an installation site for the water infiltration intake facility is limited to an ocean area where seawater is vigorously moved by currents or waves.
Accordingly, in order the solve the first problem described above, the present applicant proposed a seawater infiltration method which increases the seawater infiltration rate as high as possible under 400 m/day to thereby make it possible to intake a large amount of water within a short time, greatly reduce the filtration surface area in comparison to the prior art, and significantly reduce the scale of construction.
With regard to the second problem, if the sand filtration layer is installed in an ocean area where there is little movement of seawater by waves or currents, it is necessary to clean by artificially removing clogging substances (such as silt) which are trapped in the surface layer of the sand filtration layer. In the past, in an apparatus for artificial cleaning in the case of a river, as shown in FIG. 10, a cleaning pipe 9 having holes 9a capable of blowing air into a gravel layer 7c was embedded in order to prevent clogging of an infiltration gallery formed by backfilling the gravel layer 7c, as well as a water collection pipe 8, and a sand layer 7b under a river bed of a river 7a. 
However, if a pneumatic type cleaning pipe was used in a sand filtration layer in an ocean area where there is little movement of seawater by waves or currents, as shown in FIG. 10, clogging substances (such as silt) which are blown up into the sea above the sand filtration layer and highly concentrated turbid water containing filtration sand can drift into the area surrounding the sand filtration layer, and can become an environmental problem.
Patent Reference 1: Japanese Patent No. 3899788